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Pickering emulsion polymerized polystyrene/Fe2O3 composite particles and their magneto-responsive characteristics Young Jae Kim, Ying Dan Liu, Yongsok Seo, and Hyoung Jin Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la400523w • Publication Date (Web): 27 Mar 2013 Downloaded from http://pubs.acs.org on April 2, 2013
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Revised version for publication in “Langmuir” (2013-March)
Pickering emulsion polymerized polystyrene/Fe2O3 composite particles and their magneto-responsive characteristics Young Jae Kim1, Ying Dan Liu1,2, Yongsok Seo3 and Hyoung Jin Choi1* 1
2
Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
3
Intellectual Textile System Research Center (ITRC) and School of Materials Science and Engineering, Seoul National University, Seoul 151-747, Korea
*
Corresponding author. E-mail:
[email protected] Abstract Core-shell structured magnetic polystyrene (PS)/inorganic particles were fabricated by Pickering emulsion polymerization using nano-sized Fe2O3 particles as a solid stabilizer. Scanning electron microscopy and transmission electron microscopy confirmed the synthesized PS/Fe2O3 particles to be comprised of a PS surface coated with Fe2O3 nanoparticles. The chemical structure of the ACS Paragon Plus Environment
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composite nanospheres was characterized by Fourier transform infrared spectroscopy and X-ray diffraction. The thermal properties of composite nanospheres and corresponding pure polymer were examined by thermogravimetric analysis. The rheological properties of the core-shell structured magnetic polystyrene (PS)/inorganic particles dispersed in silicone oil were investigated under an external magnetic field strength using a rotational rheometer. The particles with extremely lower density than common magnetic particles exhibited solid-like magnetorheological phase characteristics, and the flow curves were fitted to the Cho-Choi-Jhon model of the rheological equation of state.
Introduction The applications of magnetic particles have been extended to solid dispersed substances of magnetorheological (MR) fluids, in which the MR fluids are dispersions of magnetic particles in nonmagnetic medium fluids. These fluids exhibit a reversible phase change between a liquid-like and solidlike state with an external magnetic field.1-5 Therefore, they have been applied in engineering in areas, such as engine mounts, clutches, brakes, valves and shock absorbers in the automobile industry, on account of their controllable, reversible and improved mechanical properties, and polishing characteristics.6-9 Among the range of magnetic particles available, the most popular MR material is the carbonyl iron (CI) particle. On the other hand, the high density of CI particles, which causes serious sedimentation problems, has limited their extensive engineering applications.10-12 Therefore, it is important to improve this density mismatch and sedimentation problem. One potential solution, coating the CI particles, was developed using polymeric or inorganic materials, and was also applicable to the fabrication of electroresponsive electrohreological particles.13,
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Magnetic nanoparticles of different types of iron oxides
(mostly maghemite, Fe2O3 and magnetite, Fe3O4) have been used as MR materials. In particular, Fe2O3 nanoparticles have attracted considerable interest because of their technological and fundamental
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importance, such as information storage, magnetic resonance imaging contrast agent, etc.15 Compared to the CI particles coated using a complicated process of modifying CI particles, Fe2O3 particles have a much lower density than CI particles but exhibit significant magnetic behavior.16, 17 Inorganic/organic core-shell structured particles constructed by the Pickering emulsion process have recently been introduced. In this method, the solid inorganic particles are used as a surfactant instead of conventional molecular organic surfactants.18 Nanometer-sized particles are adsorbed at the oil-water interface for stabilization because of the decrease in interfacial energy after replacing a part of the liquid−liquid or liquid−vapor interface with a liquid−solid particle interface. A range of Pickering emulsions have attracted increasing interest for practical applications in the development of novel nanostructures and microstructures,19-23 in which the solid particles used in Pickering emulsions can vary from latex particles, inorganic particles, semiconductor nanoparticles, to polymer micelles.24-26 Pickering emulsions stabilized by magnetic nanoparticles have also been reported.27-32 Recently, Yin et al. fabricated magnetite coated polystyrene microsphere using Pickering emulsion polymerization and studied its catalytic property.33 In this study, Fe2O3 coated polystyrene (PS/Fe2O3) particles were synthesized using 50 nm diameter spherical Fe2O3 nanoparticles as a stabilizer, and their MR properties were examined at various magnetic field strengths when dispersed in silicone oil. The incorporation of the polymer core and the Fe2O3 shell not only reduced the density of the magnetic particles but also improved the dispersibility of the Fe2O3 nanoparticles. The morphology of the particles obtained was found to be different from that of the commonly used CI-based composite MR particles because they are not only a core-shell structured material but also a Fe2O3 shell possessing a magneto-responsive MR effect rather than a PS core. Experimental Section Materials. Styrene (density: 0.9 g/cm3, Daejung, Korea) and 2, 2’-azobis(2-methyl-propionamidine) dihydrochloride (AIBA) (97%, Sigma-Aldrich) were used as a monomer and cationic water-soluble initiator, respectively. Fe2O3 nanoparticles (density: 5.12 g/cm3, particle size:
